How do I say 'what's up'?

怎么了 is one of the most versatile phrases in everyday Chinese — it covers casual check-ins, genuine concern, and startled reactions, all distinguished by tone:

• Light tone — “What’s up?” / “What’s going on?” A casual opener, equivalent to checking in on someone. 最近怎么了？= What have you been up to lately?
• Concerned tone — “What’s wrong?” You notice someone looks upset, quiet, or not themselves. The same words now carry worry.
• Sharp tone — “What happened?!” Something is clearly wrong and you need an explanation immediately.

The northern colloquial 咋了 (zǎ le) is worth knowing because it is everywhere online and in northern speech. 咋 replaces 怎么 in casual contexts: 咋办？(what to do?), 咋回事？(what’s going on?). Southern Chinese speakers may not use it but understand it perfectly.

怎么回事 is the fuller “what happened / what’s the situation.” It is what you ask when something obviously went wrong — a crashed car, a crying friend, a sudden silence. It expects a narrative answer, not a one-word response.